I. Field of the Invention
This invention relates to polyurethanes and more particularly to novel methods for the manufacture of load distributing, shockattenuating polyurethanes of low rebound resilience and the resulting products.
II. Background of the Related Art
Shock-attenuating polyurethanes have been used in a variety of applications which make use of an unusual combination of physical properties available in these materials. Sometimes referred to as "viscoelastic" materials, the shock-attenuating polyurethanes of this invention are like elastic materials in that, when a stress is relieved, they return to their initial shape and do not take a permanent set. They differ from true elastics, however, in that their return, while substantially complete, is not immediate. Hence the polyurethanes with which this invention is concerned display a low rebound velocity in ball rebound and pendulum impact tests. This suggests that they are well suited for use such as, shock-attenuating bumpers or buffer pads to reduce the effect of impact; noise controllers to prevent acoustic resonance; and vibration dampers to moderate vibration induced wave propagation and oscillation. The polyurethanes of this invention are also well suited to protect the human body and are finding acceptance, for example, as shock absorbing insoles for shoes and protective pads in sports.
Commonly assigned U.S. Pat. No. 4,722,946 claims a shock-attenuating polyurethane made by the process of reacting a mixture of linear and branched polyols, a polyisocyanate, and optionally a blowing agent, under polyurethane forming conditions at an isocyanate index of from about 65 to 85, the teachings of which are incorporated herein by reference.
It has now been found that shock-absorbing polyurethanes, having generally similar properties to those obtained in practicing the '946 patent, can be obtained by using branched polyisocyanates with a functionality of at least 2.3 in combination with a substantially linear polyol rather than a polyisocyanate with a combination of both linear and branched polyols.